This invention relates generally to a chromatically corrected directional diffusing screen. This invention relates particularly to a diffusing screen that is generated by a holographic technique. Still more particularly this invention relates to a diffusing screen that directs chromatically balanced light into one or more specific eyeboxes, or exit pupils, such that the screen appears to have an enhanced and uniform brightness when viewed from each eyebox.
Optical diffusers are used for improving the light uniformity in illuminating systems and for producing a directional redistribution of incident light so that an image projected on a screen may be observed from different directions. In display devices both aspects of diffusers are important since the illumination uniformity and image brightness for the required observation directions are important.
Generally, if a diffuser is illuminated with approximately collimated incident light, it transforms incident beams into a light pattern with a given angular distribution. A diffuser usually decreases the light brightness in the propagation direction while at the same time increasing the brightness for other directions. Nevertheless, it always produces lower brightness for directions different from propagation direction of illuminating beam. Therefore, application of a diffuser in a display system with side observation requires a very bright, high-power light source. In some cases, the available light is limited to the degree that it permits only poor performance of the display system. An improvement in diffuser characteristics therefore could play a very important role in increasing the quality of optical display systems.
Properties of diffusers may be described by three characteristics:
(1) Angular distribution of the diffused light when the diffuser is illuminated with a parallel beam, PA1 (2) total light losses in the diffuser, PA1 (3) for some applications, imaging properties.
These characteristics depend mainly on the physical structure and technology used in forming the diffuser. Existing diffusers may be categorized as being either volume diffusers or surface diffusers.
Volume diffusers are usually made from a transparent material which includes the light scattering centers or at least two surface diffusers set in tandem. The properties of volume diffusers strongly depend on the volume concentration of the scattering centers in the material and the thickness of the material in the first case. The properties of volume diffusers also depend on the scattering properties of particular components and distance between them.
Surface diffusers are usually made from a solid plate having a light scattering surface. Depending on the application, the substrate of the diffuser may be either transparent for transmitted light or either semitransparent or opaque for reflected light.
Fiber-optic and holographic diffusers have recently been developed. The fiber-optic diffuser redistributes the incident light by transmitting it through a short length of optical fiber. The holographic diffuser redistributes the incident light by diffraction.
The design of a diffuser depends on the application in which it will be used. Light losses are always important; therefore, nonabsorbing materials are used. If the object itself has strong diffusing properties, the most important factor is the uniformity of the illuminating light. Volume diffusers are generally preferred when uniformity of the light distribution is the most important design consideration. The imaging properties of the diffuser are directly related to its thickness. Therefore, thin volume or surface diffusers are generally preferred for imaging systems.
The performance of a display for viewing by an observer is limited by the brightness and resolution of the image as seen by a viewer in the viewing zone. In most cases from a selected point of view, the brightness of the image varies across the screen and depends on the viewer's position. The brightness of the screen is not the same over the entire screen area, and the distribution of the light depends on the viewer's position within the viewing zone. The distribution of the light on the screen may make it difficult for the viewer to observe all parts of the screen and reliably extract information therefrom. Therefore, it may be desirable to define a viewing pupil that receives the majority of light from the display.
Significant interest has been generated in the last few years in mechanisms that direct light in specific directions from the diffusing screens of display devices. Such mechanisms permit the available light to be used more efficiently by directing it specifically towards the observer so that the image appears brighter than with conventional screens. Directional diffusing screens providing better brightness uniformity across the screen from the selected point of view using holographic techniques were first discussed in the open literature by Meyerhofer, Applied Optics, Vol. 12, No. 9, pp 2180-2184, September 1973. U.S. Pat. No. 4,372,639 issued Feb. 8, 1983 to Johnson discloses a holographic diffusing screen and method of construction. The construction technique includes the use of large aperture conventional optics to image a real diffusing screen on to the hologram and also to form the eyebox, which is the exit pupil of the diffuser. The technique of Johnson is cumbersome and difficult to implement. The diffusing screen formed by this technique can be used only for monochromatic source applications. U.S. Pat. Nos. 4,586,780 and 4,586,781 disclose improvements by the use of a fiber-optic face plate to eliminate the zero order light from the holographic diffusing screen and to provide a rudimentary form of chromatic correction for some single directions.